Final answer:
Sigma bonds are formed by the head-on overlap of s and/or p orbitals between two atoms, characterized by cylindrical symmetry of electron density around the internuclear axis. Option a is correct .
Step-by-step explanation:
The orbitals that make up a sigma bond are characterized by the head-on overlap of atomic orbitals, which can include: (a) two s orbitals, (b) an s orbital and a p orbital, or (c) two p orbitals. The overlapping orbitals enable two atoms to share electrons, forming a strong covalent bond known as a sigma (σ) bond.
A sigma bond is cylindrical in its electron density around the internuclear axis. This means that if a line were drawn between the two nuclei in the bond, it would pass right through the centre of this cylindrical region of electron density. In contrast, pi (π) bonds form by side-to-side overlap and have electron density above and below the internuclear axis.